In recent years, in the field of paints, transformation from organic solvent-based paints to water-based paints is promoted from the view points of environmental protection and safety-hygiene. However, water-based paints are on a lower level with coating performance of weather resistance, water resistance, freeze-thaw resistance, and particularly stain-resistance comparing with organic solvent-based paints. Therefore, it is current that there are many problems to be solved.
Development of water-based paints and development of laminating methods of coatings directed to a solution of these problems have been performed in various ways. For example, as resins for water-based paints, emulsions of acrylic resins by emulsion polymerization are attracted attention because the obtained coating has characteristics having relatively good properties such as weather resistance. In addition, a method such that laminate coatings are performed with imparting properties to under coatings, intermediate coatings and top coatings respectively by using water-based emulsions to impart coating performance such as stain-resistance to the entire laminate material has been developed.
Technical methods of stain-resistant improvement include (i) a method of raising the hardness of a coating (glass transition temperature of a resin (Tg) is raised), (ii) a method to hydrophilize a coating surface to impart a faculty of self-purification to wash away a pollutant with rain, and (iii) a method to restrain adhering electrostatic pollution by regulating electrostatic property, and so on.
However, when a glass transition temperature is raised and the hardness of a coating is raised as in method (i), film formation characteristics deteriorate and problems such as occurring of cracks in winter (freezing damage characteristics) happen. In addition, large improvement of stain-resistance is difficult. In a method to hydrophilize a coating surface as in method (ii), there is a problem that weather resistance deteriorates as for hydrophilizing by monomer compositions. Further, as for the addition of materials such as surfactants, water resistance deteriorates, and there also has a problem in continuance of stain-resistant effect. In a method to regulate electrostatic property as in method (iii), addition of an antistatic agent gives bad influence for weather resistance and water resistance, and there also has a problem in the durability of the effect.
A method to contain inorganic materials is known as one of methods to solve the above problems and to improve properties of water-based paints. For example, in Patent reference 1, a coating composition in which colloidal silica is incorporated into a water-based emulsion by copolymerizing vinyl silanes and acrylic monomers is proposed, and has characteristics excellent in heat resistance, water resistance, and adhesion of a coating.
However, this coating composition needs large contents of colloidal silica of 500-20000 parts by mass in solid content for 100 parts by mass in solid content of the water-based emulsion. When large contents of colloidal silica are used, there have problems that transparency and weather resistance of the coating, storage stability of the coating composition, and coating workability become inferior.
On the contrary, Patent references 2 and 3 disclose that relatively small contents of colloidal silica are used as essential components.
In Patent reference 2, a flame-retardant paint material containing 100 parts by mass of a polyorgano siloxane-based graft copolymer emulsion in which 10-90 mass % of a unsaturated vinyl monomer is graft-copolymerized for polyorgano siloxane, and 1-50 parts by mass of colloidal silica is proposed. The above flame-retardant paint material has characteristics excellent in flame retardancy, extensibility, adhesiveness, temperature sensitivity, moisture permeability, and stain-resistance of the coating.
However, this paint material has problems of low stain-resistance with insufficient hydrophilic property, transparency, and hardness of the coating, because of containing large contents of a water-repellent silicone component which is 52 parts by mass or more of a polyorgano siloxane component for 100 parts by mass in solid content of the polyorgano siloxane-based graft block copolymer emulsion.
In Patent reference 3, an aqueous coating composition containing 100 parts by mass of a polyorgano siloxane-based graft copolymer emulsion and 1-300 parts by mass of colloidal silica is proposed. The aqueous coating composition is excellent in storage stability, and a coating has characteristics excellent in hardness, water resistance, and stain-resistance. However, lowering pollution by a further hydrophilization of the coating is requested although stain-resistance is largely improved as compared to the coating comprising the paint material described in Patent reference 2.
In addition, in Patent references 4 and 5, and so on, at a case of surface coating of cement-type materials, a method to prevent pollution adhesion from static electricity and to impart stain-resistance to a coating is proposed by preventing static electricity retention of a whole laminated material which has a multi-layer structure of an under coating, an intermediate coating, and a top coating, and each of all layers contains a silicate or silica sol.
However, when either one layer of the under coating, the intermediate coating or the top coating was a coating only with a resin, a static electricity retention was generated to cause a problem of pollution adhesion.
In addition, in Patent reference 6, an antifouling aqueous coating composition, which comprises an aqueous dispersion having a silicone structure containing phenyl group, a sulfo-succinic acid-type surfactant, a surfactant having alkylene oxide group and colloidal inorganic particles (colloidal silica), is proposed. However, in the antifouling aqueous coating composition described in Patent reference 6, stain-resistance in a satisfactory level has not achieved.    Patent reference 1: Japanese Examined Patent Application, Second Publication No. H1-41180    Patent reference 2: Japanese Unexamined Patent Application, First Publication No. H4-23857    Patent reference 3: Japanese Unexamined Patent Application, First Publication No. H9-165554    Patent reference 4: Japanese Examined Patent Application, Second Publication No. S53-34141    Patent reference 5: Japanese Unexamined Patent Application, First Publication No. H5-96234    Patent reference 6: Japanese Unexamined Patent Application, First Publication No. 2004-149668